The present invention relates to the general field of the gas exhaust nozzle fitted to bypass turbomachines, and it relates more particularly to a turbomachine having a stationary nozzle in which the throat section can nevertheless be varied artificially.
A bypass turbomachine comprises in particular a gas turbine engine provided with a fan disposed on a longitudinal axis of the turbomachine, and with an annular nacelle centered on the longitudinal axis of the turbomachine and surrounding the engine, the nacelle having an upstream end surrounding the fan of the engine and a downstream end forming a nozzle for ejecting the gas coming from the turbomachine. The term “throat section” is used to designate the cross-section of the nozzle constituting its smallest cross-section along the entire length of the nozzle.
It is known that by varying the throat section of the nozzle of a turbomachine, it is possible to control the flow rate through the fan so as to place the fan in operating conditions that correspond to optimum efficiency at any operating speed of the turbomachine. The use of exhaust nozzles having a section that is geometrically variable is thus common practice in military applications. The techniques used generally have recourse to flaps disposed in line with the downstream end of the outer wall of the nozzle, with the flaps being steered to reduce or increase the section of the nozzle.
Unfortunately, those techniques are difficult to implement on the nozzles of turbomachines for civilian applications. This is due in particular to constraints associated with installing the nacelle relative to the wing of the airplane, to ground clearance, and to the thicknesses and shapes of the trailing edges of the nacelle. Furthermore, such variable section nozzles are relatively expensive to fabricate.
Thus, the nozzles used in civil aircraft generally have a throat section that is geometrically fixed and optimized for cruising flight since that represents the major fraction of the mission of an airplane. As a result, nozzles of fixed throat section operate in suboptimal manner when the engine is running at high speed (corresponding to takeoff and while the airplane is climbing), and while the engine is running at low speed (corresponding to descent, to the approach stage, and to the airplane idling in flight).